1. Field of the Invention
The present invention relates generally to bearings, and more particularly to a sliding ceramic bearing which can be readily produced.
2. Prior Art
Sliding bearings and rolling bearings are popularly used in applications such as attaching a rotary axle to a machine frame, etc.
Common types of rolling bearings include ball bearings, roller bearings and needle bearings, in which rolling members such as balls, rollers, and needles are provided between an inner ring and an outer ring. The friction between the inner ring and the outer ring is known as “rolling friction,” and is generally very small. Therefore, rolling bearings provide good high-speed operating capabilities. However, the roller members between the inner ring and the outer ring are prone to crack or deform under a heavy load. When this happens, the operating precision is dramatically decreased. In addition, the manufacturing costs of roller bearings are very high, especially roller bearings used in small devices such as computer fans.
Therefore, in small devices, sliding bearings are often used because of their relatively low manufacturing costs. A typical sliding bearing comprises an annular bearing sleeve having a circular bore, and a cylindrical shaft rotating in the bore. Most bearing sleeves used today are made of a copper-based alloy or stainless steel. The friction between the bearing sleeve and the shaft is known as “sliding friction,” and is generally very large. To reduce the friction between the shaft and the bearing sleeve, a diameter of the bore of the bearing sleeve is configured slightly larger than a diameter of the shaft in order to provide an operating clearance, and an oil film is established in the operating clearance to act as a lubricant. Because of the operating clearance, the shaft is usually not located exactly along a central axis of the bearing sleeve. Instead, the shaft is displaced slightly from the central axis so that it rotates about an axis that is eccentric to the central axis. This leads to unsteady rotation of components mounted on the bearing sleeve. However, if the operating clearance is configured to have a reduced size, the lubricant therein may be forced out. When this happens, the bearing sleeve directly contacts the shaft, and the sliding bearing rapidly wears out. Therefore, sliding bearings usually have short lifetimes.
With the development of technology in fields where slide bearings are applied, modem slide bearings are being required to rotate at unprecedented high speeds. The problem of “sliding friction” is becoming commensurately more important. Traditional low abrasion, high hardness materials used for slide bearings are increasingly unable to provide satisfactory high-speed, long-life performance under harsh operating conditions. New materials for bearings are being eagerly sought. It has been found that certain ceramics have high compression strength, high friction resistance, and a small coefficient of friction. Ceramics are now widely considered to be a more serviceable material for slide bearings than traditional materials. Studies have shown that in ceramic slide bearings, it is feasible to reduce the contact area between the bearing sleeve and the shaft in order to reduce the friction therebetween, without diminishing the operating reliability of the slide bearing.
Taiwan Patent Publication No. 495118 discloses a sliding bearing made of ceramic material. In order to reduce the contact area between the bearing sleeve and the shaft, either an outer surface of the shaft or an inner surface of the bearing sleeve is configured to be non-cylindrical. When the bearing sleeve receives the shaft therein, at least a portion of the outer surface of the shaft does not contact the inner surface of the bearing sleeve, so that the contact area is reduced. However, the advantages of high compression strengthen and high abrasive resistance of the ceramic material also present novel problems in manufacturing the bearing sleeve, as detailed below.
Referring to FIG. 7, to attain a high degree of surface smoothness, a bore 4 of a bearing sleeve 1 needs to be ground with a grinding machine 2. The grinding machine 2 has a grinding bit 3 rotatingly machining the surface of the bearing sleeve 1 that defines the bore 4. During this process, the grinding bit 3 is subjected to a diametrical force by the bearing sleeve 1. This causes the grinding bit 3 to bend, especially when the grinding bit 3 is extended far into the bore 4. As a result, the ground bore 4 is irregular. That is, a diameter of the bore 4 nearest the grinding machine 2. One means of ameliorating this problem is to perform doubled-ended grinding. Referring to FIG. 8, two grinding machines 2 are provided to simultaneously grind the bore 4 at opposite ends thereof. Each grinding bit 3 has to penetrate only halfway into the bore 4. Accordingly, the grinding bits 3 are subjected to reduced diametrical forces, and the ground bore 4 is more uniform. However, it is generally not possible to completely eliminate irregularity of the bore 4. In addition, the two ground halves of the bore 4 may not be precisely coaxial, due to inherent manufacturing error.